sdk/trace: add Jaeger remote sampler

[kvrhdn]

This adds the Jaeger remote sampler.

Closes #2148

Usage:

tp := tracesdk.NewTracerProvider(
	tracesdk.WithSampler(
		tracesdk.JaegerRemoteSampler("myService", "http://localhost:5778"),
	),
	// ...
)
otel.SetTracerProvider(tp)

[kvrhdn]

Instead of importing jaegertracing/jaeger we should import jaegertracing/jaeger-idl. This is currently blowing up go.sum.
In fact, we only need proto/api_v2/sampling.proto.

I'm not sure how to do this though, help/feedback is welcome!

We should add it as git submodule:

[submodule "jaeger-idl"]
	path = sdk/trace/internal/jaeger-idl
	url = https://github.com/jaegertracing/jaeger-idl

But I'm not sure how to generate and import the proto as part of the build process.

• Do I add a target in Makefile?
• Should I check in the generated code or should people generate it themselves first?

fyi, the jaegertracing/protobuf image doesn't work on my machine (Apple M1). So that's also making testing more difficult... 😬

[Aneurysm9]

This would need to live in a separate module to ensure that the increased dependency footprint only impacts applications that choose explicitly to include this sampler. Indeed, it might be best for this to live in the contrib repo as it is not required to conform to the spec.

[kvrhdn]

Sure, sounds good 👍 There aren't any other samplers yet in contrib, should I create a new folder, i.e. samplers/jaeger_remote?

[MrAlias]

Sure, sounds good There aren't any other samplers yet in contrib, should I create a new folder, i.e. samplers/jaeger_remote?

Yeah, please create a new sampler directory. We can discuss in the contrib PR, but I would prefer samplers/jaeger.

[kvrhdn]

Closing in favour of open-telemetry/opentelemetry-go-contrib#936